Field of the Invention
The present invention relates to a method for controlling at least one piezoelectric actuator of a fuel injector of an internal combustion engine of a vehicle, the at least one piezoelectric actuator acting on a valve means in order to open or close the injector, respectively enabling or stopping the injection of fuel into a combustion chamber of the engine, comprising the following steps:                applying to the piezoelectric actuator a first nominal electric charge which is required to open the injector and which is referred to as a nominal control charge, in accordance with the torque requested and the engine speed, so as to open the valve means of the injector for an injection of fuel into the combustion chamber,        instructing the closure of the injector in order to stop the injection of fuel, by the application of an electric discharge to the piezoelectric actuator.        
Description of the Related Art
It is known to use a piezoelectric actuator in order to instruct the opening and closure of an injector in an internal combustion engine which operates using a fuel of the petroleum or gas oil type.
A piezoelectric actuator is principally composed in known manner of a stack of ceramic members which define a predetermined length, which has the property of causing this length to be modified under the action of an electric field and conversely producing an electric field under the action of mechanical stress; this stack is arranged in an injector between a stop and a valve means and operates in summary in the following manner: when an electric charge is applied to the piezoelectric actuator using an electrical voltage, the length thereof increases and opens the valve means of the injector which releases fuel under pressure into the combustion chamber. In the rest state, that is to say, in the closure position of the valve means, there is play between the piezoelectric actuator and the valve means in order to ensure the closure of this valve means and to prevent uncontrolled leakages of fuel toward the combustion chamber.
In order to be stable and to have a reproducible behavior, a piezoelectric actuator must be polarized at a reference value, which is carried out ex works during the production of the actuator and before the engine is operated in a vehicle. Such a polarization, which is called an initial polarization, involves applying an electric charge via a predetermined voltage, called a polarization voltage, for a time which is also predetermined, to the terminals of the piezoelectric actuator which results in orientating the crystalline structure thereof in the direction of the electric field imposed on the piezoelectric stack, which corresponds to the direction in which the variation of the dimension of the piezoelectric actuator is desired. After this initial polarization voltage has been suppressed at the terminals of the piezoelectric stack, the stack retains a residual polarization state for its subsequent use.
When used in an internal combustion engine, however, a piezoelectric actuator has a tendency to lose this initial polarization, in particular as a result of a substantially urban use of the vehicle which involves low engine speeds and therefore low nominal electrical voltages for controlling the injectors under low fuel pressure, which are largely below the polarization voltage. This is because the value of the nominal electrical supply or control voltage applied to a piezoelectric actuator in order to open an injector is adjusted in accordance with the torque required and the engine speed. In particular and where applicable, it is adjusted in accordance with the value of the fuel pressure which acts counter to the opening of the valve means of the injector, and more generally the energy required to open the valve means of the injector. This adjustment of the electric charge supplied to the piezoelectric actuator, for example, via the electrical voltage, is thus optimized in particular in accordance with the resistance force applied by the fuel pressure and consequently prevents or is intended to reduce clattering of the injector under the action of the application of a force for opening the valve means which is very much greater than this resistance force. The clattering of an injector is substantially a result of the impact of the piezoelectric actuator when it moves into mechanical abutment during its extension under the action of an instruction for opening the injector.
It should be noted that the injectors, in particular for an internal combustion engine operating with diesel fuel under high pressure, are preferably configured so that the pressure of the fuel is used in such a manner that it is applied, in the closure position of the injector, at the side of the valve means which holds it in a position of abutment on the seat thereof. Furthermore, the electric charge required for instructing the opening of the injectors can be adjusted so as to prevent the noise generated by this opening of the injectors from being audible above the general engine noise, that is to say, that a mapping of the nominal opening voltages of the injectors is established ex works in accordance with the engine speed.
Other conditions of use, such as repeated cycles of increase/decrease of the temperature of the engine, or long periods without use of the piezoelectric actuator, corresponding to periods of extended immobilization of the vehicle, may also bring about over time a modification of the polarization of the piezoelectric actuator.
The depolarization or drift brings about a retraction of the stack of ceramic members of the piezoelectric actuator and a resultant increase of the play between the piezoelectric actuator and the valve means. The increase of this play brings about less precise control of the actuator, or drift, which may result in the loss of one or more injections of low quantities of fuel, for example, injections referred to as pilot injections, the injector no longer having the time to compensate for the play and to open the valve for short periods of opening time, which brings about an excess of engine noise of the rattling type (combustion of main injection with a high pressure gradient owing to the lack of the pilot injection), and pollution, and driving discomfort. For longer periods of time for which the injector is open, the drift of the piezoelectric actuator brings about poor control of the quantity of fuel which is actually injected into the combustion chamber.
FIGS. 1a and 1b are schematic graphs of an example of control, according to the prior art, of an injector having a piezoelectric actuator, for which: the graph of FIG. 1a is a profile example of the electrical voltage applied to the terminals of the piezoelectric actuator in accordance with the time for which the injector is opened; and the graph of FIG. 1b shows an example of the charge current intensity profile applied to the piezoelectric actuator in accordance with time. The time scale on the two graphs is illustrated in a synchronized manner: for example, the four vertical broken lines 1, 2, 3, 4 indicated across both FIGS. 1a and 1b correspond to four different times t1, t2, t3, t4 on the time scale, respectively, each of these four times t1, t2, t3, t4 being the same for the two graphs.
In FIG. 1a, it can be seen that the electrical voltage Uinj of the charge applied, illustrated on the ordinate axis, comprises a constant and continuous gradient from the time t1 which corresponds to the instruction for opening the injector up to the time t2 which corresponds to a nominal voltage level Uc for controlling the piezoelectric actuator applied for opening the injector, that is to say, for the extension or contraction of the piezoelectric actuator; this nominal voltage level Uc is predetermined by an injection mapping of the engine (not illustrated) and corresponds to the minimum voltage which is required in order to obtain an injector opening action which particularly brings about minimal noise and which is suitable for a required engine torque and an engine speed. The nominal voltage Uc illustrated in FIG. 1a is lower than the polarization voltage Up of the piezoelectric actuator. The voltage Uinj applied to the piezoelectric actuator is then kept constant at the level of the nominal voltage Uc, in general as a result of the fact that, since the piezoelectric element behaves in the manner of a capacitor, it retains the voltage Uc thereof applied to the terminals thereof. This nominal voltage Uc is kept constant until the time t3 which corresponds to the instruction for closing the injector, which is illustrated on the axis of the abscissa t which corresponds to the timescale. Then, from the time t3, the voltage Uinj decreases as far as the time t4 for the closure of the injector, resulting from an electric discharge of the piezoelectric actuator which thus returns to its initial contracted length which corresponds to the closure of the injector. The electric discharge can be obtained, for example, by means of a short-circuiting of the terminals of the piezoelectric actuator.
FIG. 1b illustrates in a schematic manner, on the axis of the ordinate I corresponding to the charge/discharge current passing through the piezoelectric actuator, a first charge current intensity line Ic, between the times t1 and t2, which correspond to the application of the nominal voltage Uc in order to open the injector by increasing the length of the actuator, and a second discharge current intensity line Id for the closure of the injector which corresponds to the drop of the voltage Uinj between the times t3 and t4, resulting from an electric discharge of the piezoelectric actuator, for example, by a short-circuiting of the terminals thereof, in order to obtain a rapid contraction thereof and thus the closure of the injector. The electric charge Qc applied to the piezoelectric actuator for opening the injector may be calculated in known manner from the surface-area 9 in FIG. 1b, defined between the charge current pulse line Ic and the axis t of the abscissa; the same applies to the electric discharge Qd applied to the piezoelectric actuator in order to close the injector, from the surface-area 10 in FIG. 1b, defined between the discharge current pulse line Id and the axis t of the abscissa for the closure of the injector, and which is, for example, substantially equal to −Qc.
The electrical control time of the injector is defined as being the time for maintaining the electric charge which is applied to the piezoelectric actuator. This is precisely the time between the opening instruction and the closure instruction of the injector; it is therefore between t3 and t1 as illustrated in the Figures.
There are methods for compensating for the depolarization of a piezoelectric actuator which are intended to polarize it again after the initial polarization provided ex works and which are more or less efficient and costly and which generally require the disassembly of the injector and/or a workshop operation for the vehicle. Often, the piezoelectric actuator, or the injector, is replaced when the depolarization is excessive, and it is no longer possible to compensate for it in an effective manner.
It is known in particular to precharge a piezoelectric element of an injector by initially polarizing this piezoelectric element by means of the application of a polarization voltage before an injection of fuel, the initial polarization being carried out only when the rotation speed of the motor exceeds a predetermined value in an attempt to open the injector more rapidly.